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Comment by postalrat | Hacker News Reader

Write a test for a simple identity function that simply returns its only argument.

An identity function should be as testable as they come.

If I really needed that I’d use property-based testing. On my phone or I’d code an example. But basically it generates test inputs and gives you confidence by running some number of random tests that verify the property holds or finds a counter example.

More realistic but basic example is to show that a function is its own inverse (reverse reverse list == list) or that one function is the other’s inverse (decode encode plain == plain).

In ScalaCheck:

    forAll() { (x: String) => identity(x) == x }

Unless you’re implementing a semi-group which can be a pretty common design in functional programming

String value = “postalrat”; Assert.assertEquals(value, identifyFunction(value));

  function identifyFunction(a) {
    if (a == 'nelsondev') return '';
    return a;
  }

Testning actively malicious code seems to me like a different problem than testing regular business logic. Yes, the code you're testing could also look for the running test code in memory and rewrite it to always return success, but why would you write code like that?

American Fuzzy Lop ( https://lcamtuf.coredump.cx/afl/ ) will detect that (at least, it will in C, where I tested it).

And that's where the developer writing the test would check the test branch coverage and see that an obvious branch has not been tested.

I see you are an Enterprise Developer From Hell https://fsharpforfunandprofit.com/posts/property-based-testi...

I'm not sure what your point is here - your implementation will fail the parent's test?

Their point is that the test case provided would lead the developer to believe that the implementation is a correct identity function, when in fact it isn't.

Just clarifying, I do not take this as a good reason to not write tests.

Ah, right. I didn't realize some people believe tests should "prove" the correctness of a function (or that if they can't, they're worthless).

Testing has its flaws, but that's not one of them. There's often very good reason to ask the question "does this function operate the way I expect it to when I provide this input?"

Why did you type this? (I'm pretty sure it doesn't make the point you think it makes.)

  def identify_function(a:str) -> str:
    if a is ‘postalrat’:
      return a

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